Table Of Contents: Building And Using Static And Shared "C" Libraries
What Is A "C" Library? What Is It Good For?
Creating A Static "C" Library Using "ar" and "ranlib"
Using A "C" Library In A Program
Creating A Shared "C" Library Using "ld"
Using A Shared "C" Library - Quirks And Solutions
Using A Shared "C" Library Dynamically - Programming Interface
Loading A Shared Library Using dlopen()
Calling Functions Dynamically Using dlsym()
Unloading A Shared Library Using dlclose()
Automatic Startup And Cleanup Functions
Getting a Deeper Understanding - The Complete Linking Story
The Importance Of Linking Order
Static Linking Vs. Dynamic Linking

Building And Using Static And Shared "C" LibrariesOne of the problems with developed programs, is that they tend to grow larger and larger, bringing up overall compilation and linking time to a large figure, and polluting out makefile, and the directory where we placed the source files. The first time a program we write reaches this state, is normally when we look for a different way to manage our projects.

It is this point where we start thinking about combining out source code into small units of related files, that can be managed with a separate makefile, possibly by a different programmer (for a multi-programmer project).

What Is A "C" Library? What Is It Good For?One of the tools that compilers supply us with are libraries. A library is a file containing several object files, that can be used as a single entity in a linking phase of a program. Normally the library is indexed, so it is easy to find symbols (functions, variables and so on) in them. For this reason, linking a program whose object files are ordered in libraries is faster then linking a program whose object files are separate on the disk. Also, when using a library, we have fewer files to look for and open, which even further speeds up linking.

Unix systems (as well as most other modern systems) allow us to create and use two kinds of libraries - static libraries and shared (or dynamic) libraries.

Static libraries are just collections of object files that are linked into the program during the linking phase of compilation, and are not relevant during runtime. This last comment seems obvious, as we already know that object files are also used only during the linking phase, and are not required during runtime - only the program's executable file is needed in order to run the program.

Shared libraries (also called dynamic libraries) are linked into the program in two stages. First, during compile time, the linker verifies that all the symbols (again, functions, variables and the like) required by the program, are either linked into the program, or in one of its shared libraries. However, the object files from the dynamic library are not inserted into the executable file. Instead, when the program is started, a program in the system (called a dynamic loader) checks out which shared libraries were linked with the program, loads them to memory, and attaches them to the copy of the program in memory.

The complex phase of dynamic loading makes launching the program slightly slower, but this is a very insignificant drawback, that is out-weighted by a great advantage - if a second program linked with the same shared library is executed, it can use the same copy of the shared library, thus saving a lot of memory. For example, the standard "C" library is normally a shared library, and is used by all C programs. Yet, only one copy of the library is stored in memory at any given time. This means we can use far less memory to run our programs, and the executable files are much smaller, thus saving a lot of disk space as well.

However, there is one drawback to this arrangement. If we re-compile the dynamic library and try to run a second copy of our program with the new library, we'll soon get stuck - the dynamic loader will find that a copy of the library is already stored in memory, and thus will attach it to our program, and not load the new (modified) version from disk. There are ways around this too, as we'll see in the last section of our discussion.

Creating A Static "C" Library Using "ar" and "ranlib"The basic tool used to create static libraries is a program called 'ar', for 'archiver'. This program can be used to create static libraries (which are actually archive files), modify object files in the static library, list the names of object files in the library, and so on. In order to create a static library, we can use a command like this:

ar rc libutil.a util_file.o util_net.o util_math.o

This command creates a static library named 'libutil.a' and puts copies of the object files "util_file.o", "util_net.o" and "util_math.o" in it. If the library file already exists, it has the object files added to it, or replaced, if they are newer then those inside the library. The 'c' flag tells ar to create the library if it doesn't already exist. The 'r' flag tells it to replace older object files in the library, with the new object files.

After an archive is created, or modified, there is a need to index it. This index is later used by the compiler to speed up symbol-lookup inside the library, and to make sure that the order of the symbols in the library won't matter during compilation (this will be better understood when we take a deeper look at the link process at the end of this tutorial). The command used to create or update the index is called 'ranlib', and is invoked as follows:

ranlib libutil.a

On some systems, the archiver (which is not always ar) already takes care of the index, so ranlib is not needed (for example, when Sun's C compiler creates an archive, it is already indexed). However, because 'ar' and 'ranlib' are used by many makefiles for many packages, such platforms tend to supply a ranlib command that does nothing. This helps using the same makefile on both types of platforms.

Note: when an archive file's index generation date (stored inside the archive file) is older then the file's last modification date (stored in the file system), a compiler trying to use this library will complain its index is out of date, and abort. There are two ways to overcome the problem:

Use 'ranlib' to re-generate the index.
When copying the archive file to another location, use 'cp -p', instead of only 'cp'. The '-p' flag tells 'cp' to keep all attributes of the file, including its access permissions, owner (if "cp" is invoked by a superuser) and its last modification date. This will cause the compiler to think the index inside the file is still updated. This method is useful for makefiles that need to copy the library to another directory for some reason.

Using A "C" Library In A ProgramAfter we created our archive, we want to use it in a program. This is done by adding the library's name to the list of object file names given to the linker, using a special flag, normally '-l'. Here is an example:

cc main.o -L. -lutil -o prog

This will create a program using object file "main.o", and any symbols it requires from the "util" static library. Note that we omitted the "lib" prefix and the ".a" suffix when mentioning the library on the link command. The linker attaches these parts back to the name of the library to create a name of a file to look for. Note also the usage of the '-L' flag - this flag tells the linker that libraries might be found in the given directory ('.', refering to the current directory), in addition to the standard locations where the compiler looks for system libraries.

For an example of program that uses a static library, try looking at our static library example directory.

Creating A Shared "C" Library Using "ld"The creation of a shared library is rather similar to the creation of a static library. Compile a list of object files, then insert them all into a shared library file. However, there are two major differences:

Compile for "Position Independent Code" (PIC) - When the object files are generated, we have no idea where in memory they will be inserted in a program that will use them. Many different programs may use the same library, and each load it into a different memory in address. Thus, we need that all jump calls ("goto", in assembly speak) and subroutine calls will use relative addresses, and not absolute addresses. Thus, we need to use a compiler flag that will cause this type of code to be generated.
In most compilers, this is done by specifying '-fPIC' or '-fpic' on the